This invention relates to electrocardiography and, more particularly, to means for analyzing a portion of an electrocardiographic signal to predict potential ventricular tachycardias.
Sudden death from acute arrhythmia is a major risk in the first few hours after a myocardial infarction. During the first few days, the incidence of ventricular arrhythmia is approximately 90%. The percentage of arrhythmias decreases considerably after the first several days but still presents a substantial risk to the myocardial infarct patient. Statistically, without treatment, approximately 50% of all infarct patients will eventually die of ventricular arrhythmia.
A reproducible and consistent ability to predict a patient's propensity for lapsing into an arrhythmia is needed. Several investigators, employing signal averaging techniques, have detected, on the body surface, small, high frequency potentials in the late QRS and ST-segments of electrocardiograms in patients and animals prone to ventricular tachycardia. (Uther, et al.: "The Detection of Delayed Activation Signals of Low Amplitude in the Vector Cardiogram of Patients with Recurrent Ventricular Tachycardia by Signal Averaging", In Management of Ventricular Tachycardia--Role of Mexiletine, edited by E. Sandoe, et al., Excerpta Medica, Amsterdam, 1978, pp. 80-82.) Drs. Uther, et al. found that these potentials did not occur in healthy, young people and suggested that they represented areas of delayed myocardial depolarization.
Obviously, if it can be shown that the high frequency signal in the late QRS of a myocardial infarct patient is common to most, if not all, infarct patients who are subject to ventricular tachycardia, an important new diagnostic tool would would be available. Technically, however, it is extremely difficult to isolate accurately high frequency signals late in the QRS complex. A filter must be used to eliminate the lower frequency portions. Unfortunately, substantially all filters "ring" for a period of time after application of the relatively high energy, initial portion of the QRS waveform. This ringing effectively hides any low amplitude, high frequency portions late in the QRS.
In a large clinical trial supervised by the inventor, using an electrocardiographic analysis system which will be hereinbelow described, it has been found that 92% of postmyocardial infarct patients who are subject to ventricular tachycardia, do, indeed, exhibit a distinctive high frequency signal tail in their late QRS signal. This signal is present in only 7% of post infarct patients who are free of ventricular tachycardias. In addition, it was found that a patient subject to ventricular tachycardia will exhibit a QRS signal of substantially longer duration than patients without ventricular tachycardia.
Accordingly, it is an object of this invention to provide an electrocardiographic system, capable of reliably determining the presence of absence of a high frequency segment during the late portion of a patient's QRS signal and measuring the magnitude of that segment.
It is another object of this invention to provide an electrocardiographic analysis system which is capable of determining the width of a QRS signal.